New York City ? Look for Rin Tin Tin on the R train.

The New York Police Department has deployed the first class of K-9 units devoted exclusively to the subway, the city’s top transit officer said Monday.

“We have four out there now, with another three to follow by mid-December,” said James Hall, chief of the NYPD’s Transit Bureau. “They won’t work anywhere else but the subway.”

The dogs and their handlers began patrolling the trains Nov. 14. The last K-9 unit dedicated completely to mass transit was disbanded in the 1980s, but Police Commissioner Ray Kelly revived the idea in January.

“The biggest issue here is deterrence,” said Lt. John Pappas, commanding officer of the Transit Bureau K-9 Unit. “The overall mission is counterterrorism and to fight crime.”